In some known solar power systems, a plurality of photovoltaic (PV) panels (also known as solar panels) are logically or physically grouped together to form an array of solar panels. The solar panel array converts solar energy into electrical energy. The electrical energy may be used directly, converted for local use, and/or converted and transmitted to an electrical grid or another destination.
Solar panels generally output direct current (DC) electrical power. To properly couple such solar panels to an electrical grid, or otherwise provide alternating current (AC) power, the electrical power received from the solar panels is converted from DC to AC power. At least some known solar power systems use a single stage or a two-stage power converter to convert DC power to AC power. Some such systems are controlled by a control system to maximize the power received from the solar panels and to convert the received DC power into AC power that complies with utility grid requirements.
DC to AC converters typically need energy storage because the instantaneous input power is DC, and hence is constant when measured over periods of tens to thousands of milliseconds, whereas the output power is a time varying AC output. Energy storage is used to store energy when the AC output power is lower than the DC input power and to release energy when the AC output power is higher than the DC input power. Many known systems use electrolytic capacitors as a main energy storage element in DC to AC converter designs.
FIG. 6 is a graph (referred to as “10”) that shows a DC energy input 12 and an AC energy output 14 from an energy storage element as a function of time for an ideal lossless DC to AC converter operating into a single phase AC power grid at 60 Hertz and rated at 250 Watts. As can be seen in graph 10, the difference between the two curves is a power ripple at twice the AC grid frequency, 120 hertz (Hz) ripple in 60 Hz AC power systems.
In some known DC/AC converters, energy storage electrolytic capacitors are placed at the input to the converter. The energy storage component does not have to be located at the input to the converter, but may instead be located somewhere in the middle of the power conversion process. For example, some DC/AC systems are designed to have a DC/DC conversion stage followed by DC link storage followed by a DC/AC output conversion stage. When electrolytic capacitors are used at the input of the DC/AC converter or in a DC link, they are typically sized to maintain a low ripple voltage, both at whatever switching frequency the inverter operates but also at the ripple frequency of twice the AC power frequency (e.g., 120 Hz in 60 Hz power systems).
Electrolytic capacitors are a relatively low cost approach to getting relatively large amounts of storage capacitance. However, electrolytic capacitors wear out relatively quickly, limiting their useful lifetime. Electrolytic capacitors are typically rated at 5,000 to 10,000 hours of full power operation. The typical wear out mechanism in electrolytic capacitors is the evaporation of the electrolyte in the capacitor, which causes internal resistance to increase and capacitance to decrease. Electrolytic capacitors also have relatively high internal resistance, potentially resulting in relatively high power losses and reduced efficiency of the converters in which they are included. Accordingly, a better solution is needed.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.